1. Field of the Invention
This invention relates to a nickel alloy useful in a heater for a glow plug to be mounted on an internal combustion engine such as a diesel engine.
2. Description of the Prior Art
In recent years, automobiles with diesel engines have increased in number. Because diesel engines have the defect that when the engine is cold, a longer time for starting is requried and it is desired to shorten this time. For this purpose, a rapid heating-type glow plug whose temperature is increased by passing a large electric current through the heater must be used. It is important for (a) such a rapid heating-type glow plug to have a large positive coefficient of electrical resistance-temperature to shorten the time for the temperature increase and to prevent breaking physically by melting due to excessive heat; and (b) such a property to be maintained for long periods of time and the coarsening of the crystal grains at high temperatures and high temperature degradation thereof by expansion and shrinkage incident to heating and cooling to be minimized. To meet requirement (a), the heater of the glow plug should have an electrical resistance value at about 800.degree. to 1000.degree. C. (i.e., the practical temperature range for heaters) at least three times higher than that at about 20.degree. C. When a pure metal such as pure nickel (denoting nickel usually containing less than 0.5% by weight of manganese, silicon and cobalt and less than 0.1% by weight impurities) is used as the heater, the crystal grains of nickel become coarser at high temperatures, and thus degradation of the nickel occurs. Moreover, such a pure metal is susceptible to oxidation and is not durable. Also, the strength of such a pure metal at high temperatures is degraded, and breaking tends to occur. Thus, this heater cannot meet requirement (b) mentioned above. When active additive such as chromium is mixed with pure nickel to form a solid solution, the durability of the metal can be retained, but the ratio of the electrical resistance at high temperatures to that at room temperature is below 3, and rapid heating cannot be achieved.